My pie dish has a diameter of 9 inches/ 23 cm, so I rolled the dough into a circle of about 10 inches/ 26 cm in diameter. It is loose-bottomed (like a springform), which makes things easier for me when removing the pie from the form. A regular dish works as well, but you will have to be more careful when taking the tart out of the form. Or use a ceramic pie dish, so you can cut and serve the tart directly from the dish.
Preheat the oven to 400 degrees Fahrenheit/ 200 degrees Celsius.
Roll dough: On a lightly floured surface roll out the larger disc into a circle large enough to cover the bottom and the edges of your pie form. Place the rolled-out pastry sheet into your dish, pressing the dough into the fluted edges of the pan.
Lattice: Roll out the second disc of pastry into a circle of about 1/8 inch/ 3 mm thickness and a diameter of about 9 inches/ 23 cm. Cut the pastry into long strips for the lattice topping.
Harry Potter decoration: If you want to decorate the treacle tart to resemble Harry Potter, cut out the glasses, the scar, and the mouth. You will have leftover pastry in this case. You can cut it into biscuits and bake them separately until golden. Or freeze the leftover pastry for making a small pie some other time.
Assemble tart: Pour the filling into the prepared pie dish and level the top. Lay half of the strips over the filling in one direction and the other half of the strips in the other direction to form a lattice. Trim the edges.
Egg wash: Mix the egg and the water and brush the lattice and the edges of the pie with the mixture.
Bake the tart for 10 minutes at 200 degrees/ 400 degrees Fahrenheit, then reduce the heat to 180 degrees Celsius/ 350 degrees Fahrenheit and continue baking for another 25 minutes until the crust is golden brown and the filling puffs up in the center. The filling should still be slightly jiggly.